This invention relates to an image-forming system containing an array of electromechanical grating devices. More particularly, the invention relates to the formation of gray levels in a projection display system containing a linear array of electromechanical grating devices.
Many projection display systems employ a spatial light modulator to convert electronic image information into an output image. At present in such display systems, the light source is typically a white light lamp and the spatial light modulator is typically an area array containing liquid crystal devices or micromirror devices. Alternative display system architectures with one or more laser sources and spatial light modulators that are linear arrays of electromechanical grating devices show much promise for the future. To display high quality motion images, the individual devices of these different spatial light modulators must be capable of rapidly producing a large number of gray levels in the image. The limit on the number of possible gray levels is usually dictated either by the device dynamics or by the speed of electronic components within the display system.
Prior inventions have disclosed schemes for increasing the number of gray levels in the image without increasing the speed of the modulating elements or of the associated electronics. These schemes vary the illumination that is incident on the spatial light modulator during a frame. Specifically, according to U.S. Pat. No. 5,812,303, issued to Hewlett et al. on Sep. 22, 1998, entitled, xe2x80x9cLIGHT AMPLITUDE MODULATION WITH NEUTRAL DENSITY FILTERS,xe2x80x9d additional gray levels can be obtained with a micromirror device by using a variable neutral density filter to generate bright and dark gray levels. The dark gray scale is obtained by attenuating the illumination for some time during the display of a frame. The bright gray scale has no attenuation. In practice, the invention is implemented by rotating a filter wheel with a multi-segment neutral density filter in synchronization with the data stream.
An alternative approach employs a pulsating light source, such as a pulsed laser to reduce speed requirements on the electronic components, as described in U.S. Pat. No. 5,668,611, issued to Ernstoff et al. on Sep. 16, 1997, entitled xe2x80x9cFULL COLOR SEQUENTIAL IMAGE PROJECTION SYSTEM INCORPORATING PULSE RATE MODULATED ILLUMINATION.xe2x80x9d The illumination on the spatial light modulator is adjusted by varying the pulse rate or pulse count. Moreover, the average brightness of the light source is determined by the number of pulses. A complementary method uses direct intensity modulation of the light source to obtain multiple levels of brightness, as disclosed in U.S. Pat. No. 5,903,323, issued to Ernstoff al. on May 11, 1999, entitled xe2x80x9cFULL COLOR SEQUENTIAL IMAGE PROJECTION SYSTEM INCORPORATING TIME MODULATED ILLUMINATION.xe2x80x9d Both U.S. Pat. No. 5,668,611 and U.S. Pat. No. 5,903,323 address the specific problem of having a large enough time window for the electronic components to load new image data bits into the spatial modulator.
Each of the above described inventions trade light source efficiency for improved gray levels or reduced electronic component speed requirements. However, efficient use of the light source is needed for high-quality projection displays in order to maximize brightness and color saturation of the projected image.
Recently, an electromechanical conformal grating device consisting of ribbon elements suspended above a substrate by a periodic sequence of intermediate supports was disclosed by Kowarz in U.S. Pat. No. 6,307,663, issued to Kowarz on Oct. 23, 2001, entitled xe2x80x9cSPATIAL LIGHT MODULATOR WITH CONFORMAL GRATING DEVICE.xe2x80x9d The electromechanical conformal grating device is operated by electrostatic actuation, which causes the ribbon elements to conform around the support substructure, thereby producing a grating. The device of ""663 has more recently become known as the conformal GEMS device, with GEMS standing for grating electromechanical system. The conformal GEMS device possesses a number of attractive features. It provides high-speed digital light modulation with high contrast and good efficiency. In addition, in a linear array of conformal GEMS devices, the active region is relatively large and the grating period is oriented perpendicular to the array direction. This orientation of the grating period causes diffracted light beams to separate in close proximity to the linear array and to remain spatially separated throughout most of an optical system and enables a simpler optical system design with smaller optical elements. Display systems based on a linear array of conformal GEMS devices were described by Kowarz et al. in U.S. Pat. No. 6,411,425, entitled xe2x80x9cELECTROMECHANICAL GRATING DISPLAY SYSTEM WITH SPATIALLY SEPARATED LIGHT BEAMS,xe2x80x9d issued Jun. 25, 2002 and by Kowarz et al. in U.S. Pat. No. 6,476,848, entitled xe2x80x9cELECTROMECHANICAL GRATING DISPLAY SYSTEM WITH SEGMENTED WAVEPLATE,xe2x80x9d issued Nov. 5, 2002.
The conformal Grating Electromechanical System (GEMS) devices disclosed in U.S. Pat. No. 6,307,663 are illustrated in FIGS. 1-3. FIG. 1 shows two side-by-side conformal GEMS devices 5a and 5b in an unactuated state. The conformal GEMS devices 5a and 5b are formed on top of a substrate 10 covered by a bottom conductive layer 12, which acts as an electrode to actuate the devices 5a, 5b. The bottom conductive layer 12 is covered by a dielectric protective layer 14 followed by a standoff layer 16 and a spacer layer 18. On top of the spacer layer 18, a ribbon layer 20 is formed which is covered by a reflective layer and conductive layer 22. The reflective and conductive layer 22 provides electrodes for the actuation of the conformal GEMS devices 5a and 5b. Accordingly, the reflective and conductive layer 22 is patterned to provide electrodes for the two conformal GEMS devices 5a and 5b. The ribbon layer 20, preferably, comprises a material with a sufficient tensile stress to provide a large restoring force. Each of the two conformal GEMS devices 5a and 5b has an associated elongated ribbon element 23a and 23b, respectively, patterned from the reflective and conductive layer 22 and the ribbon layer 20. The elongated ribbon elements 23a and 23b are supported by end supports 24a and 24b, formed from the spacer layer 18, and by one or more intermediate supports 27 that are uniformly separated in order to form equal-width channels 25. The elongated ribbon elements 23a and 23b are secured to the end supports 24a and 24b and to the intermediate supports 27. A plurality of square standoffs 29 is patterned at the bottom of the channels 25 from the standoff layer 16. These standoffs 29 reduce the possibility of the elongated ribbon elements 23a and 23b sticking when actuated.
A top view of a four-device linear array of conformal GEMS devices 5a, 5b, 5c and 5d is shown in FIG. 2. The elongated ribbon elements 23a, 23b, 23c, and 23d are depicted partially removed over the portion of the diagram below the line Axe2x80x94A in order to show the underlying structure in an active region 8. For best optical performance and maximum contrast, the intermediate supports 27 should preferably be completely hidden below the elongated ribbon elements 23a, 23b, 23c, and 23d. Therefore, when viewed from the top, the intermediate supports 27 should not be visible in the gaps 28 between the conformal GEMS devices 5a-5d. Here, each of the conformal GEMS devices 5a-5d has three intermediate supports 27 with four equal-width channels 25. The center-to-center separation A of the intermediate supports 27 defines the period of the conformal GEMS devices in the actuated state. The elongated ribbon elements 23a-23d are mechanically and electrically isolated from one another, allowing independent operation of the four conformal GEMS devices 5a-5d. The bottom conductive layer 12 of FIG. 1 can be common to all of the conformal GEMS devices 5a-5d. 
FIG. 3a is a side view, through line 3,7-3,7 of FIG. 2, of two channels 25 of the conformal GEMS device 5b (as shown and described in FIGS. 1 and 2) in an unactuated state. FIG. 3b shows the same view for an actuated state. For operation of the device, an attractive electrostatic force is produced by applying a voltage difference between the bottom conductive layer 12 and the reflective and conductive layer 22 of the elongated ribbon element 23b. In, the unactuated state (see FIG. 3a), with no voltage difference (V=0), the ribbon element 23b is suspended flat between the supports. In this state, an incident light beam 30 is primarily reflected into a 0th order light beam 32, as in a simple planar mirror. To obtain the actuated state, a voltage is applied to the conformal GEMS device 5b, which deforms the elongated ribbon element 23b and produces a partially conformal grating with period A. FIG. 3b shows the device 5b (as shown and described in FIGS. 1 and 2) in the fully actuated state with the applied voltage at V=VHIGH and the elongated ribbon element 23b in contact with standoffs 29. The height difference between the bottom of element 23b and the top of the standoffs 29 is chosen to be approximately xc2xc of the wavelength xcex of the incident light. The optimum height depends on the specific conformal shape of the actuated device. In the actuated state, the incident light beam 30 is primarily diffracted into the +1st order light beam 35a and xe2x88x921st order light beam 35b, with additional light diffracted into the +2nd order 36a and xe2x88x922nd order 36b. A small amount of light is diffracted into even higher orders and some light remains in the 0th order. In general, one or more of the various beams can be collected and used by an optical system, depending on the application. When the applied voltage is removed, the forces due to tensile stress and bending restores the ribbon element 23b to its original unactuated state, as shown in FIG. 3a. 
FIG. 4 illustrates a display system containing a linear array 85 of conformal GEMS devices, as disclosed in U.S. Pat. No. 6,411,425. Light emitted from a source 70, preferably a laser, is conditioned by a pair of lenses 72 and 74, before hitting a turning mirror 82 and illuminating the linear array 85. The display system forms an entire two-dimensional scene from a scan of a one-dimensional line image of the linear array 85 across the screen 90. The conformal GEMS devices of the linear array 85 are capable of rapidly modulating incident light to produce multiple lines of pixels with gray levels. The controller 80 selectively activates the linear array 85 to obtain the desired pixel pattern for a given line of a two-dimensional scene. If a particular conformal GEMS device is not actuated, it reflects the incident light beam primarily into the 0th order light beam, which is directed back towards the source 70 by the turning mirror 82. If a particular conformal GEMS device is actuated, it diffracts the incident light beams primarily into +2nd, +1st, xe2x88x921st and xe2x88x922nd order light beams. These diffracted light beams pass around the turning mirror 82 and are projected on the screen 90 by the projection lens system 75. The scanning mirror 77 sweeps the line image across the screen 90 to form the two-dimensional image. Synchronization between the sweep of the scanning mirror 77 and the image data stream is provided by the controller 80.
In order to form gray levels in an image, pulse-width-modulated (PWM) waveforms are applied to the conformal GEMS devices of the linear array 85, as described by Kowarz et al., xe2x80x9cConformal Grating Electromechanical System (GEMS) for High-Speed Digital Light Modulation,xe2x80x9d IEEE 15th International MEMS Conference Technical Digest, pgs. 568-573 (2002). FIG. 5 shows a conventional single-level PWM waveform 45 with voltage VHIGH, together with the corresponding device""s output (e.g., diffracted light intensity). To obtain a desired gray level, the controller 80 selects the voltage pulse width in each modulation window 54, according to a data stream. When the single-level PWM waveform 45 transitions from 0 V to VHIGH, the device (for example, 5a and 5b shown in FIG. 1) actuates and begins diffracting light. When the waveform transitions back to 0 V, the device stops diffracting light. This process is applied to each conformal GEMS device of the linear array 85. In the display system of FIG. 4, the modulation window 54 corresponds to the time used to form a single line of the two-dimensional image. The gray level perceived in a pixel of the image is, therefore, the integrated light intensity 52 within the modulation window 54. To minimize charging effects within the device, the applied voltage can be periodically switched between VHIGH and xe2x88x92VHIGH (see U. S. Pat. No. 6,144,481). Since the force on the ribbon elements 23a-23d is independent of polarity, the diffracted light intensity is polarity independent.
FIG. 6 shows an example of a gray scale 55 generated using the conventional single-level PWM approach of FIG. 5. In this plot, relative gray levels are shown as a function of pulse width (xc2x1VHIGH) for a modulation window 54 of 20 xcexcsec. When the pulse width is between 0.3 xcexcsec and 20 xcexcsec, the relationship between gray level and pulse width is approximately linear. A non-monotonic (and nonlinear) region 50 is present, however, for pulse widths between approximately 0.3 xcexcsec and 0.1 xcexcsec. This non-monotonic behavior occurs because of the resonant dynamics of the conformal GEMS device The particular shape of the non-monotonic region 50 depends on a number of factors, including device geometry and driver slew rate. For pulse widths shorter than approximately 0.1 xcexcsec, there is a monotonic, non-linear correspondence between pulse width and gray level. The dark (gray) levels in the non-monotonic region 50 can be difficult to use in practice because of the difficulty in determining the exact correspondence between a desired gray level and the pulse width required to generate that particular gray level.
The techniques described in the prior art for improving gray levels all lower the average optical power incident on the spatial light modulator for some period of time, thus generating multiple illumination levels corresponding to decreased intensity. Multiple illumination levels reduce the speed requirements of the spatial light modulator and its associated electronic components. However, a serious technical drawback to this approach is that it wastes optical power that is available from the light source during lower illumination intervals. Furthermore, for certain types of light sources, reducing the illumination level increases system complexity. The same drawbacks apply when these techniques are used in a display system based on electromechanical grating devices. There is a need, therefore, for a method of generating enhanced gray levels in an electromechanical grating display system that makes efficient use of available optical power and does not significantly increase system complexity.
The aforementioned need is met according to the present invention by providing a method for actuating an electromechanical grating device that has ribbon elements. The method includes the steps of: providing a data stream;
generating a pulse width modulated waveform including at least two different non-zero voltages from the data stream; and applying the pulse width modulated waveform to the ribbon elements such that the ribbon elements transition through at least three different states of actuation that correspond to the pulse width modulated waveform.
Another aspect of the present invention provides a system for actuating an electromechanical grating device that has ribbon elements. The system includes: a data source that provides a data stream; means for generating a pulse width modulated waveform including at least two different non-zero voltages from the data stream; and means for applying the pulse width modulated waveform to the ribbon elements such that the ribbon elements transition through at least three different states of actuation corresponding to the pulse width modulated waveform.
A third aspect of the present invention provides an electromechanical grating device that includes a substrate and a set of elongated ribbon elements suspended above the substrate. Additionally, the electromechanical grating device includes means for generating a pulse width modulated waveform that has at least two different non-zero voltages from a data stream and means for applying the pulse width modulated waveform to the elongated ribbon elements such that the ribbon elements transition through at least three different states of actuation that correspond to the pulse width modulated waveform.
As described below, in addition to reducing the clock rate requirements, the present invention has the added benefit of allowing better generation of the dark levels corresponding to the non-monotonic region 50 of FIG. 6.